Blower assemblies that utilize tangential or cross flow fans are known. These types of blower assemblies typically include a housing and a fan rotatably mounted in the housing. The fan has two or more discs that support a number of forward-curved blades configured to draw air into the housing, across the fan, and discharge the air from the housing with rotation of the fan. Although this type of blower may have only a few components, the flow of air through these blowers is quite complex. For example, near the cutoff of the housing, the fan may develop an eccentric vortex or eddy of airflow that circulates within a portion of the fan interior near the cutoff of the housing. Further, the airflow patterns in the blower assembly are unstable and vary unpredictably which affects the efficiency and operation of the fan, and the motor driving the fan, during operation of the blower assembly.
The complexities of the airflow patterns within blower assemblies utilizing cross flow fans produce a number of problems for blower manufacturers. For example, it may be desirable to minimize the aerodynamic noise a blower makes during start-up and operation of the blower. One type of aerodynamic noise from a blower is high-pitch noise produced by blades of the fan traveling past the cutoff of the housing. The fan blades traveling past the cutoff generate sharp velocity gradients in the airflow which generates a whine or whistling sound to the human ear.
Blowers using cross flow fans also produce a lower-pitch noise as the fan, and the motor driving the fan, speeds up and slows down in response to the unstable, changing airflow patterns in the blower assembly and the resulting changing air load on the fan. The changing speed of the fan may cause the fan to sound like it is revving up or slowing down rather than rotating at a fixed speed. The somewhat constant revving up and slowing down may be unpleasant to some users.
For example, some blowers utilize electric motors having a saddle-shaped speed-torque performance curve. As the torque required to rotate the fan changes (due to the changing air load from the unstable airflow patterns), the motor operating point moves along the speed-torque performance curve in response to the changing torque requirements. The motor's saddle-shaped speed-torque curve, however, means that at a particular moment during operation of the motor there may be two operating points along the speed-torque curve that will satisfy the air load required by the fan at that particular moment. This is problematic because if the motor is operating at a first operating point with a first speed and torque combination that satisfies a particular air load, then the air load deviates briefly and returns to its original value, the motor may “search” to a second operating point with a different speed but a similar torque as the first operating point. This “searching” from the first operating point to the second operating point produces an audible revving up or slowing down of the fan as the motor changes its position along the speed-torque curve, which may be undesirable in some applications.